Although Brazil’s energy-hungry economic boom has driven the nation to tap the Amazon’s vast hydroelectric potential, the resulting deforestation accompanying its dam projects has reduced the available waterpower

The Amazon Basin is the epicenter of the world’s hydropower plants—the same gushing rains that give the region its lush foliage make it a prime destination for developers seeking to capitalize on this allegedly renewable energy source. But the long-term sustainability of these projects, which use the natural flow of water to generate electricity, is now under scrutiny.

A new study of the Belo Monte Dam, one of the world’s largest hydropower energy complexes currently under construction on the Xingu River in the eastern region of the basin, found that large-scale deforestation in the Amazon poses a significant threat to a dam’s energy-generating potential.

Although many studies have examined the impacts of deforestation on the immediate vicinity of hydropower projects, less attention has been paid to its effects on a regional scale. In fact, earlier studies found that a loss of trees within the water basin of hydropower sites increased the energy-generating capacity of the dam in the short-term, because less trees were available to suck water from the ground and export it outside the watershed in a process known as evapotranspiration.

But across an entire region, less foliage means less rainfall, so rivers flow less powerfully.

In their study, published in Proceedings of the National Academy of Sciences, researchers in the U.S. and Brazil found that large-scale deforestation in the Amazon had profound effects on the region’s water cycle—and on its climate. A loss of 40 percent of Amazonian rainforest, the scientists predicted, would reduce regional precipitation by up to 43 percent between July and October, prolonging the area’s dry season. Deforestation would thereby reduce river water discharge—assuming zero forest loss, river water surges for five months between February and June. But if 40 percent of the region’s trees were cleared, that window of heavy flow would narrow, running only from March until about May. Essentially, “the peaks get tighter,” says Michael Coe, a senior scientist at the Woods Hole Research Center’s Amazon Program in Falmouth, Mass., who worked on the study. Further, the April peak in river discharge would fall by approximately 33 percent.

So, regardless of whether hydropower developers push for increased conservation in the Xingu Basin, the study suggests they will have to take into account the effects of regional deforestation on the energy-generating capacity of their projects. “You can do a really good job conserving forest in one location,” Coe says, “but you might be undermined by activities occurring elsewhere.”

Researchers estimate that if tree-clearing practices continue as projected, the Belo Monte project could see its energy generation potential slashed by as much as 38 percent.